Recently and continuing, optical disk drive apparatuses such as CD-R drives are put to practical use, and research for enlarging storage capacity at higher speeds is being conducted. As rewritable disks, a write-once optical disk using a coloring-matter (dye) system medium, a recordable opto-magnetic medium, a recordable phase-change medium, and the like are in use.
Generally, an optical disk recording apparatus uses a semiconductor laser as the luminous source, a laser beam from which is pulse-modulated by information to be recorded, and is irradiated to a recording medium such that a recording mark is formed. At this time, the formation state of the recording mark varies with the power of the laser beam to record. For this reason, prior to starting recording the information to be recorded, a trial writing is performed to a predetermined area (PCA: Power Calibration Area, i.e., trial writing area) at various recording power levels, and a power level that provides the best reproduction signal is selected as the optimum recording power level. This process is called OPC (Optimum Power Control). Then, information recording is carried out using the optimum recording power level.
There are several criteria for evaluating the quality of such reproduction signal, i.e., for determining which signal is the best. A couple of typical criteria are described as follows.
A first criterion is characterized by evaluating such reproduction signal based on the asymmetry β of the reproduction signal (often called “the β method”). According to the β method, with reference to FIG. 3, a positive side peak value A (=Ipk−Idc) and a negative side peak value B (=Idc−Ibt) in reference to a DC level Idc of the reproduction signal are detected.
Then, β is obtained according to a formula β={(Ipk−Idc)−(Idc−Ibt)}/(Ipk−Ibt), and a reproduction signal, β value of which meets a predetermined value, for example, zero, is determined to be the best reproduction signal.
A second criterion is characterized by using a modulation index m of a reproduction signal (often called “the γ method”). According to the γ method, a peak (the maximum) value Ipk and a bottom (the minimum) value Ibt of the reproduction signal are detected as shown in FIG. 3.
Then, the modulation index m is calculated according to a formula m=(Ipk−Ibt)/Ipk. Then, a change rate γ, which is a rate of change of m to change of recording power P, is calculated according to a formula γ=(Δm/ΔP)×(P/m). A recording power level Pt that makes γ to be equal to a predetermined value γt is obtained. Then, the optimum power level is obtained by multiplying Pt and a predetermined constant k.
Nevertheless, a mark edge recording method is often used with optical disks, such as CD and DVD. According to the mark edge recording method that is suitable for high-density recording, the length of a mark carries information. According to this method, exact control of the form and the edge position of the mark is required in order to correctly reproduce data. Further, a multi-pulse recording method is often used, wherein each recording mark is formed by two or more recording pulses such that the shape of the recording marks is made uniform even if the lengths of the marks differ. That is, heating and cooling cycles are repeated, and a uniform long mark is formed by connecting marks corresponding to the pulses. This method is applied also to the coloring-matter system write-once type media.
Furthermore, various recording methods are proposed in response to demands for higher-speed and larger-capacity recording. One of such proposals uses multiple recording power levels. The multiple power level recording method is conceived for compensating for certain characteristic differences of a mark having a certain length from other marks having different lengths. For example, relations between the recording power Pw and a gap Δ from a predetermined ideal value of a recording mark having a certain length are different from the relations of other recording marks having different lengths depending on relations between the recording medium and recording pulse shape. FIG. 5 shows an example. In FIG. 5, (1) represents properties of a mark having a length equivalent to 3T, and (2) represents properties of other mark lengths. Here, T represents the reference clock cycle of data. As for CD, mark lengths range from 3T to 11T, and the gap Δ from the predetermined ideal value due to variation of the recording power as for a 3T mark is different from other marks having different lengths. In view of this, 3T marks are recorded at a power level different from other marks having other lengths such that all marks are correctly recorded according to the multiple power level recording method. In the example shown in FIG. 5, the 3T mark is recorded at the recording power Pwex, and other marks are recorded at the recording power Pw.
Conventionally, in the multiple power level recording method, when trial writing is carried out for OPC, power levels are varied such that the recording power Pwex of the specific mark length (e.g., 3T) is defined as proportional to the recording power Pw (i.e., Pwex/Pw=constant), or alternatively, the difference between Pwex and Pw is kept constant (i.e., Pwex−Pw=constant). Then, the optimum power is determined.
However, the OPC performed in this manner poses a problem due to the fact that the relations between the recording power and the gap from the predetermined ideal value vary with mark lengths. Further, when there are variations from recording medium to recording medium, and from recording apparatus to recording apparatus (namely, variation of the record pulse shapes by the variations of semiconductor laser drive units), the relations between the optimum values of the recording power for the specific mark length Pwex(opt) and the recording power for other marks Pw(opt) also vary. For this reason, it becomes impossible to obtain proper recording power levels Pwex(opt) and Pw(opt), accuracy of the mark form and the mark position is spoiled, and as a result, a problem arises in that a data error occurs.